1. Field of the Invention
The present invention relates to devices and techniques used to determine the analyte content in a fluid. More particularly, the present invention relates to systems and methods for capturing analytes on a test bed for subsequent analysis in a test device. More specifically, the present invention relates to devices and methods for determining hydrocarbon content in water.
2. Description of the Prior Art
There is a need for a new fast and economical hydrocarbon in water measurement technique that directly measures the oil content of water and does not require the use of any solvents. Infrared absorption measurements have been the preferred basis of measurement for over twenty years. However, these measurements require first performing a liquid-liquid extraction to remove the hydrocarbon from the water. The preferred solvents for performing the extraction, such as Freon, S-316, and perchloroethylene have been banned or are being phased out due to environmental, health, and safety concerns. The sensing and detection industry response to this challenge has been to introduce new methods and instruments not based on IR absorption.
As used herein, “hydrocarbon” means all molecules containing hydrogen and carbon; examples include aliphatic and aromatic molecules as well as carboxyl groups in carboxylic acids or ester groups. As used herein, “oil” means a mixture of aliphatic hydrocarbons with generally between seven and 40 carbons in the chain, aromatic species, and other hydrocarbons. It includes crude oil, refined oil, heating oil, and any other form of carbon-based oil.
The current method for measuring hydrocarbons in water approved by the Oslo-Paris Convention (OSPAR) for use in Europe and Scandinavia is Gas Chromatography-Flame Ionization Detection (GC-FID) (OSPAR Commission Reference Number 2005-15). The method requires the use of solvent (pentane is recommended) to perform a liquid-liquid extraction for sample preparation. This method has the advantage of directly measuring the oil content and differentiating TPH from BTEX and Grease. However, GC-FID is extremely time consuming and labor intensive, requiring up to an estimated 6 hrs per measurement and many more for periodic recalibrations. Also, the differentiation of TPH from Grease content is not inherent in the measurement technique but instead requires separate sample preparation by an experienced operator.
As used herein, “TPH” means Total Petroleum Hydrocarbons, generally including non-volatile aliphatic molecules of varying chemical structure with up to 40 carbons. As used herein, “BTEX” stands for all aromatic organic molecules, including Benzene, Toluene, Ethylbenzene, and ortho-, meta- and para-Xylene. As used herein, “Grease” refers to long chain hydrocarbon molecules containing carboxylic acid and/or ester functional group or groups.
The current US standard method for measuring hydrocarbons in water approved by the Environmental Protection Agency (“EPA”) (EPA 1664) to replace the previous IR-based methods (EPA 418.1 and 413.2) is also based on liquid-liquid extraction. Simply, after extracting the oil from the water into a solvent, generally hexane, the hexane is evaporated and total mass of material remaining is measured and reported as the TPH or TOG (as used herein, “TOG” means Total Oil and Grease; that is, the total of TPH and Grease and excluding BTEX). The EPA 1664 method also introduced new terminology specific to the method. Instead of TOG, EPA 1664 refers to Hexane Extractable Material, or HEM. Instead of TPH, EPA 1664 refers to Silica Gel Treated Hexane Extractable Material, or SGT-HEM. Differentiating TOG (or HEM) from TPH (or SGT-HEM) requires separate sample preparation by the operator. This method is also labor intensive and the measurement takes a long time. It must be ensured that there is no water present and all the hexane is evaporated, as the presence of either will result in over-reporting the TPHI/TOG content of the sample. This means one measurement can take up to 48 hrs. In a revision to EPA 1664, the EPA has promulgated EPA 1664A, a technique that allows solid phase extraction (SPE) of the HEM from water using SPE discs or cartridges, followed by the elution of the HEM from the SPE material with hexane. As in EPA 1664, the hexane is then evaporated from the sample and the remaining material is weighed to determine HEM. SGT-HEM is determined by re-dissolving the HEM in hexane to perform the silica gel treatment. While EPA 1664A reduces the amount of solvent required and the time to perform the test, it cannot be used on certain samples due to clogging issues and does still require significant solvent use (about 200 ml of hexane per test) and time (about 1.5 hrs for most samples).
Other competing measurement techniques are based on the ultraviolet fluorescence, ultraviolet absorbance, or simultaneous spectral ultraviolet fluorescence/absorbance of the BTEX components of the oil content. They have the advantage of being capable of measuring very low amounts (as low as 50 ppb has been claimed) of BTEX in water and measuring the sample in water with no liquid-liquid extraction sample preparation step. However, since this method is based on measuring just the aromatic (BTEX) component of the sample, the presence of TPH and/or Grease must be determined by calibration of the expected oil stream by some method that can measure all three components. This issue is a significant drawback when performing measurements for regulatory compliance, which generally require the measurement of all the polluting components of the aqueous sample of interest, and when unknown oil contaminant streams are encountered.
Light scattering/turbidity is the other major non-IR based technique in use for oil in water analysis. This technique relies on the fact that oil is very slightly soluble in water (generally below 1 ppm) and so it is actually a two-phase system, i.e., oil is present as droplets in water. These droplets scatter light of certain wavelength depending on the droplet size and the intensity of the scattering at a certain wavelength depends on both the number of droplets and droplet size. Therefore, the number and size of oil droplets can be measured by examining the light scattering profile of the flowing two phase fluid system. However, problems are encountered with gas bubbles and solid particles also scattering light, thus leading to overestimating the oil content of the sample. The walls of such a device must be transparent to the wavelength range of interest at the point the measurement is performed. However, oil and other potential contaminates in the sample will tend to rapidly foul all surfaces, necessitating thorough cleaning after relatively short periods of operation.
Other methods, such as those based on ultrasonic acoustic pulse echo, are unproven and highly complex and thus unlikely to find wide acceptance.
German Patent No. DE2754293 describes a particular extraction solvent for use in automated systems available from HORIBA, Ltd, of 2 Miyanohigashi, Kisshoin, Minami-ku Kyoto 601-8510 Japan. These systems were designed for use to comply with EPA 418.1, and so are essentially made obsolete by the banning or phasing out of most extraction solvents. While these systems use the infrared radiation absorbing property of hydrocarbons as the basis for sensing oil in water, they require the use of solvent for liquid-liquid extraction.
The standard practice worldwide generally required the use of chlorofluorocarbon solvents, which are harmful to the ozone layer and have generally been banned worldwide, or other extraction solvents, such as perchloroethylene, which are hazardous to the health and safety of the operator and are also being phased out worldwide. Therefore, the solvent-based systems are generally obsolete in practice. Some other systems provide for the capture and regeneration of the extraction solvent for reuse, but this is generally considered insufficient environmentally.
U.S. Pat. No. 5,109,442 describes a hydrophobic material such as Teflon® (available from the DuPont Company of Delaware) that is used solely as a waterproofing component and not as a hydrocarbon-absorbing material as in the present invention, but the use of that system containing Teflon® material for oil in water measurement is not described. In general, the absorptive film consists of a metal having a refractive index that changes when in contact with various analytes. This metal film is coated on an optical fiber through which light of some unknown frequency is passed, but which cannot be infrared radiation due to the fiber optic material. The change in refractive index of the cladding results in a change in the light signal exiting the optical fiber which is correlated to the concentration of analyte in the gas or liquid being measured. Therefore, this technique does not directly measure the oil content, but instead measures a change in a secondary material property (refractive index) of the cladding. Also, it is explicitly stated that platinum cladding responds strongly to the BTEX components, so in effect the sensing methodology is twice removed from directly measuring the oil content. That is, the device is measuring a secondary material property response to only a small portion of the total hydrocarbon content in the water. The technique therefore relies on calibrations of the total hydrocarbon content relative to the content of BTEX compounds which is often unknown and or changing with time.
In “Determination of oil and grease by sold phase extraction and infrared spectroscopy”, Analytica Chimica Acta 395 (1999) 77-84), Ferrer and Romero describe a method which requires a vacuum filtration apparatus to perform the oil separation from water. A vacuum filtration method fails to supply sufficient pressures to ensure fluid flow in a timely manner (i.e. <10 minutes) through a membrane due to filter clogging. This limitation is significant since real-world samples typically contain high levels of metals/metal oxide particles, organic materials, and other particulates which clog and consequently inhibit fluid flow though the membrane unless sufficiently high differential pressures across the membrane are applied.
The Romero method further requires the membrane to be physically handled and extracted from the vacuum filtration apparatus, then re-attached to a different membrane holder via magnetic supports for post-collection IR analysis. Among other things, this can lead to undesirable collector contamination and delays in the analysis process. These and other limitations of the Romero method as described in the noted reference result in a system that is not adequate for commercialization.
Ferrer and Romero further describe another system for the determination of hydrocarbons in water in “Fourier Transform Infrared Spectroscopy and Solid Phase Extraction Applied to the Determination of Oil and Grease in Water Matrices,” Microchemica Acta 140, 35-39(2002), which consists of a vaporizing hydrocarbons out of the water sample and onto a PTFE disc suspended above the water surface. The method is recommended by the authors mainly for use on diesel and petrol-containing samples, as the processing conditions (heat and time, up to 14 hours in some cases) and calibration to be used vary considerably with the type of hydrocarbon present in the water sample. The described method thus is not widely applicable or commercially viable.
While the description of the prior art has been directed to the determination of hydrocarbon content in water, it is to be noted more generally that Based on the foregoing, there is a need for a commercially suitable apparatus and related method to detect analytes in fluids with reasonable accuracy. In general, it is desirable to have an analyte determination apparatus and method that effectively retains accurate and reliable samples of the analyte for evaluation using known evaluation tools including, but not limited to, IR spectroscopy.